1. Field of the Invention
This invention relates to an apparatus and a method for mixing gaseous fuel and air for combustion, and particularly for the mixing of gaseous fuel and air in a premixing combustion air passage of a combustor of a gas turbine. While the apparatus and method of the invention can be applied to mixing of fuel and air for various purposes, explanation will primarily be made below in relation to gas turbine combustors or burners.
2. Description of the Related Art
Gas turbine burners which can operate in two combustion modes have become generally adopted. JP-A-61-22127 (corresponding to U.S. Pat. No. 4,898,001) discloses a gas turbine burner which employs diffusion combustion using multiple nozzles and premixing combustion also using multiple nozzles. Low temperature combustion using excess air is generally performed, in order to reduce NOx production. However, an extremely wide range of fuel supply rate is required from ignition up to rated load, in gas turbine combustion, making it impossible to cover this broad range wholly by premixing combustion. Therefore it is necessary to employ diffusion combustion over a range of combustion rate from ignition up to a certain speed of rotation of the gas turbine or to a certain load level.
In diffusion combustion, there is a tendency for high temperature areas to occur locally, leading to a higher emission level of NOx. Therefore it is desirable to switch to premixing combustion, giving a uniform and low temperature combustion with excess air, as soon as possible in order to reduce NOx. Accordingly, startup of the gas turbine is effected with diffusion combustion at the time of ignition, and then the burner is gradually switched to premixing combustion, with support from the diffusion combustion flame, when the ratio of air and fuel reaches the limit for premixing combustion.
Particularly at low fuel supply rates, even in premixing combustion, low NOx production may not be achieved at all times. What is required is to achieve a uniform mixing of the fuel into the premixing combustion air.
JP-A-61-22127 mentioned above describes the supply of fuel locally into the premixing combustion air flow through a plurality of nozzles, but it has been found that concentration distribution of the fuel in the air is inconsistent, when changes occur in the volume of air flow or velocity of fuel injection. In particular, the locus of the injected fuel, after injection into the air may change considerably, with change of load. Uneven fuel distribution leads to higher NOx production.
JP-A-62-294815 shows injection of fuel from a nozzle located centrally in an air flow passage, at a straight portion of the passage. Reliance is therefore placed upon mixing of the fuel and air as they pass along the passage, but no special measures are taken.
In the construction shown in JP-U-59-108054, fuel is injected in a radially outward direction with respect to the access of the burner into a venturi region of an air flow passage. The fuel is injected from a radially inner cylindrical wall portion of the air passage towards a convexly curved radially outer wall portion of the passage. Here again reliance is placed on mixing of the air and fuel downstream of the fuel injection region, in an approximately straight part of the air passage.